1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus having an X-ray shield, and more particularly to a removal of a scattered line by detecting an amount of the scattered line from a region in which a first-order X-ray is shielded by the X-ray shield in a state in which the X-ray shield is located between a focal point of an X-ray source and an X-ray detection device.
2. Related Background Art
In recent years, an X-ray sensor capable of directly converting an X-ray image into a digital output in real time has been proposed. For example, it is possible to manufacture a solid light detection device in which solid light detection elements, each of which is composed of a transparent conductive film and a conductive film, are arranged in two-dimensional matrix on a substrate made of quartz glass with an amorphous semiconductor film interposed therebetween. Thus, the X-ray sensor is an X-ray detection device in which the solid light detection device and a scintillator that converts X-rays into visible lights are laminated.
Regarding a process that obtains an X-ray digital image in the case where the X-ray detection device is used, when the X-ray detection device is irradiated with the X-rays that pass through an object, the X-rays are converted into the visible lights by the scintillator and the converted visible lights are detected as electrical signals by photoelectric conversion portions of the solid light detection elements.
The electrical signals are read from the respective solid light detection elements by a predetermined reading method. An A/D conversion is performed on the read electrical signals to obtain X-ray image signals. The detection device is described in detail in Japanese Patent Application Laid-Open No. H08-116044.
Also, a large number of detection devices that directly obtain X-rays by the solid light detection device without using the scintillator have been proposed. Those detection devices have no light scattering resulting from the scintillator as compared with the case of the detection device using the scintillator. Accordingly, it is generally said that the resolution is preferable.
Further, a large number of detection devices in which a CCD or a CMOS detection device is combined with the scintillator to increase the number of times image taking is performed per unit time have been proposed. Hereinafter, such an X-ray sensor capable of directly converting an X-ray image into a digital output in real time is called an X-ray detection device.
An X-ray computed tomography apparatus using the X-ray detection device has been proposed.
In a conventional X-ray computed tomography apparatus, a one-dimensional X-ray detection device in which detection elements are arranged in one-dimensional line is used to rearrange only a cross-section of an object, thereby obtaining a slice image of the object. In the case where a two-dimensional X-ray detection device in which detection elements are arranged in two-dimensional matrix is used, not only the slice image of the object but also a three-dimensional image thereof can be obtained.
In the case where the one-dimensional X-ray detection device is used, X-rays radiated from an X-ray source are collimated so as to form a fan beam according to a width of the one-dimensional X-ray detection device. In contrast to this, in the case where the two-dimensional X-ray detection device is used, X-rays radiated from the X-ray source may be collimated according to a size of the two-dimensional X-ray detection-device. Accordingly, it is generally said that the X-rays form a cone beam.
Hereinafter, an X-ray computed tomography apparatus which uses the fan beam X-rays and includes the one-dimensional X-ray detection device is called a fan beam CT. In addition, an X-ray computed tomography apparatus which uses the cone beam X-rays and includes the two-dimensional X-ray detection device is called a cone beam CT.
Regarding the fan beam CT, in the case where a three-dimensional image of an object is obtained, it is necessary that scanning of the object and its vicinities is conducted plural times to obtain a plurality of slice images and the obtained slice images are combined with one another. Therefore, a large amount of heat is generated in the X-ray source to apply a large amount of load to the X-ray source. In addition, an image taking time required for obtaining the three-dimensional image is lengthened, so that the burden is put upon a patient as the object.
Regarding the cone beam CT; although depending on the size of the two-dimensional X-ray detection device, the three-dimensional image of the object can be obtained by conducting scanning on the object and its vicinities only once. Accordingly, the image taking time can be shortened and the load to the X-ray source can be reduced.
Note that a part of X-rays entered onto the object is scattered. Therefore, in addition to first-order X-rays directly entered from the X-ray source onto the X-ray detection device through the object, the scattered X-rays also enter onto the X-ray detection device and are thus detected.
The amount of scattered X-ray is about 20% of the total detection amount in the case of the fan beam CT. In contrast to this, in the case of the cone beam CT, the amount of scattered X-ray is equal to or larger than half of the total detection amount. Therefore, there is a problem in that the accurate image rearrangement is impossible.